Inhibition of corrosion in aqueous systems

ABSTRACT

The present invention provides an effective method of inhibiting corrosion on metallic surfaces in contact with a fluid contained in an industrial fluid system, which includes adding to such fluid an effective corrosion controlling amount of a particular generally recognized as safe (GRAS) compound.

FIELD OF THE INVENTION

The present invention relates generally to the protection of metallicsurfaces from corrosion in both the vapor and liquid phases of aqueousand non-aqueous fluid systems. More specifically, the present inventionrelates to corrosion inhibiting compositions and methods of using thesame.

BACKGROUND OF THE INVENTION

Corrosion of metallic components in plants may cause system failures andsometimes plant shutdowns. In addition, corrosion products accumulatedon the metal surface will decrease the rate of heat transfer between themetal surface and the water or other fluid media, and thereforecorrosion will reduce the efficiency of the system operation. Therefore,corrosion can increase maintenance and production costs.

The most common way to combat corrosion is to add corrosion inhibitingadditives to the fluid of such systems. However, many currentlyavailable corrosion inhibiting additives are either non-biodegradable,toxic, or both, which limits the applicability of such additives.

The most common anti-corrosion additives used in connection with boilercondensate systems are neutralizing amines and filming amines. Whileamines and combinations of amines generally provide effective protectionagainst the corrosion of steel and other ferrous-containing metals, theuse of amines in anti-corrosion additives presents several problems.

First, amines often undergo thermal decomposition at high temperaturesand form ammonia, which can be very corrosive to copper and copperalloys, especially in the presence of oxygen. Thus, amine-containingcorrosion inhibitors are often unsatisfactory for use in systemscontaining copper or copper alloy metallurgies.

Further, in a number of applications including food processing, beverageproduction, co-generation plants, and pharmaceutical manufacturing, theuse of amines is limited due to governmental regulations or concerns fortaste and odor problems. Consequently, in many of these applications, noanti-corrosion treatment program is used at all. Therefore, thesesystems are susceptible to high corrosion rates, significant maintenancecosts and high equipment failure rates.

U.S. Pat. No. 5,368,775 discusses methods of controlling acid inducedcorrosion. In one method, a thin film is used as a barrier between themetal surface to be protected and the acidic solution. Long chain aminessuch as octadecyl amine or azoles are used to form the thin film. Thesecond method requires the addition of neutralizing amines to neutralizethe acid and raise the aqueous pH. The best amines for this method aredescribed as having a high basicity and a low molecular weight.Cyclohexylamine, dimethylamine, trimethylamine, morpholine, andmethoxypropylamine were cited as examples of neutralizing amines.

U.S. Pat. No. 4,915,934 discloses a foamable biocide compositioncomprising an alcoholic chlorohexidine solution, quick breaking foamingagent, an aerosol propellant, and corrosion inhibitor to counter thecorrosive nature of the alcoholic chlorohexidine solution. The quickbreaking foaming agent contains, as one of its ingredients, a surfaceactive agent, preferably an ethoxylated sorbitan ester. The surfaceactive agent acts as an emulsifier. Examples of the preferred emulsifiergiven include ethoxylated sorbitan stearate, palmitate, and oleate;nonyl phenol ethoxylates; and, fatty alcohol ethoxylates.

U.S. Pat. No. 3,977,994 discloses a rust inhibiting composition. Thecomposition is a mixture of an organic acid, an N-alkyl or cycloalkylsubstituted ethanolamine, and water. In some cases, the composition mayalso contain at least one emulsifying agent to permit the emulsion ofthe organic acid and the ethanolamine. Examples of the emulsifying agentinclude sorbitan derivatives.

U.S. Pat. No. 4,970,026 teaches a corrosion inhibitor for ferrous andnon-ferrous aqueous systems. The composition comprises a componentselected from a naphthenic oil based sodium salt of a triethanolaminealkylsulfamido carboxylic acid; a paraffinic oil based sodium salt of atriethanolamine alkylsulfamido carboxylic acid; a sodium salt of analkylsulfamido carboxylic acid; and a mixture consisting of two choicesas well as a surfactant selected from a long chain fatty acid derivativeof sarcosine and a condensation product of ethylene oxide and a fattyacid.

The inhibiting effects are attributed to the component or mixture ofcomponents, not to the addition of the surfactant. In fact, the patentstates that the surfactants were tested separately for theireffectiveness as corrosion inhibitors. The surfactants were found to beineffective as corrosion inhibitors.

U.S. Pat. No. 5,082,592 discloses a method for inhibiting corrosion forferrous metals in aqueous solution comprising a nonionic surfactant andan anionic oxygen containing group such as alkali metal salts of borate,molybdate, and nitrate/nitrite. The preferred nonionic surfactant isphenol/polyethylene oxide.

It is postulated in the specification that the nonionic surfactantincreases the corrosion inhibition properties of the anions. Theinhibition properties of the anions result from their adsorption at theinterface of the metal surface and the solution. It is believed that theco-absorption of the nonionic surfactant serves to maximize the surfaceconcentration of the anions by shielding the anions' hydrostaticrepulsive forces.

EPO Patent Application, No. 0 108 536 B1 discloses a method forprotecting metal surfaces from corrosion. The method uses a compositionof a corrosion inhibitor with a thickening agent. The corrosioninhibitor may include carboxylic acid esters of sorbitan. In combinationwith a thickening agent, the corrosion inhibitor is pseudoplastic andthixotropic. The composition forms a gel upon standing. The compositionforms a soft, flexible coating which can replace paints, varnishes,lacquers, plastics and metal coatings frequently used to protect metalsurfaces from corrosion.

Therefore, there is a strong need for a corrosion-inhibiting non-amine,less toxic additive which is a more environmentally acceptablealternative. In the present invention particular generally recognized assafe (GRAS) substances surprisingly provide protection of metallicsurfaces from corrosion in aqueous and non-aqueous solutions.

SUMMARY OF THE INVENTION

The present invention provides an effective method of inhibitingcorrosion on metallic surfaces in contact with a fluid contained in anindustrial fluid system, which comprises adding to such fluid aneffective corrosion controlling amount of a generally recognized as safe(GRAS) compound. The GRAS compound may be, e.g., an acyloin oralpha-hydroxyketone, riboflavin (also known as Vitamin B₂, flavin,lactoflavine, ovoflavin or 7,8-dimethyl-10-ribitylisoalloxazine),diallyl disulfide (a component of garlic and garlic oil) or cysteine.

In a preferred embodiment of the invention, the compound is diallyldisulfide. The acyloin compound may be, e.g., acetoin, or3-hydroxy-2-butanone. The acetoin compound and its oxidation product,2,3-butanedione, are both GRAS-listed chemicals. Riboflavin, diallyldisulfide and cysteine are also GRAS-listed chemicals. Thisclassification makes the materials more readily acceptable for their useas metal corrosion inhibitors in food industry applications.

The compositions of the present invention should be added to the fluidsystem for which corrosion inhibition activity of the metal parts incontact with the fluid system is desired, in an amount effective for thepurpose. This amount will vary depending upon the particular system forwhich treatment is desired and will be influenced by factors such as thearea subject to corrosion, pH, temperature, water quantity andrespective concentrations in the water of corrosive species. For themost part, the present invention will be effective when used at levelsof from about 0.025-50 parts per million (ppm) of fluid, and preferablyfrom about 0.05-10 ppm of fluid contained in the system to be treated.The present invention may be added directly to the desired fluid systemin a fixed quantity and in a state of an aqueous solution, continuouslyor intermittently. The fluid system may be, e.g., a cooling water,boiler water, boiler steam, steam condensate, gas scrubbing or pulp andpapermaking system. Other examples of fluid systems which may benefitfrom the treatment of the present invention include heat transfersystems, refinery systems, food and beverage systems, and mechanicalcoolant systems.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will now be further described with reference to a numberof specific examples which are to be regarded solely as illustrative andnot as restricting the scope of the present invention.

Testing was performed in a laboratory corrosion test apparatus. Theapparatus included a source of deionized and deoxygenated water, ahigh-pressure pump, a series of metering pumps to deliver the materials,and associated sensors. The testing apparatus for the present inventionincluded a 16 foot mild steel coil (OD: 0.25“, ID: 0.135“) that was usedas the corroding metal. Details of the experiments and conditions are asfollows: 1. deionized carbonated water as feedwater; 2. oxygen added tothe feedwater at an oxygen concentration of 10 ppb; 3. flow of 180ml/min inside the mild steel coil; 4. temperature of the coil and theflowing solution was maintained at 104° C.±3° C.

For better thermal stability, the coil was housed in a heated can filledwith sand. Two internal thermocouples monitored the coil's inlet/outlettemperatures. The concentration of CO₂ in the carbonated water wasmaintained constant and measured with a carbon analyzer, Sievers TOC800. The pH of the solution was estimated at 5.15±0.10.

The total iron concentration in the fluid at the outlet of the apparatuswas representative of the corrosion in the mild steel coil. The ironconcentration was roughly estimated colorimetrically in the laboratorywith 1,10 phenanthroline as reactive, and the total iron content wasdetermined by inductive coupled plasma (ICP).

The percentage corrosion inhibition was calculated as the difference intotal iron concentration at the outlet of the coil between the untreatedcoil (Fe ppm UT) at the conditions of the run and the total ironconcentration exiting the chemically treated coil (Fe ppm T) at the sameexperimental conditions:

-   -   % Corrosion Inhibition=((Fe ppm UT)−(Fe ppm T))×100)/(Fe ppm UT)

The kinetic of the iron release was followed for the untreated coil andfor some treated runs. The untreated iron coil equilibrated quickly, in4 to 8 hours. The following results were for equilibration times of20-26 hours for each concentration. During that time, in order to findthe kinetics of the iron release, outlet fluid samples were taken andanalyzed for total iron content. Calculations of the percent inhibitiongiven by the chemical were carried out using the iron totalconcentration measured at the end of the set equilibration time (20-26hours).

Before and after each run, the iron coil was disconnected from theapparatus, activated, reconnected, and allowed to reach equilibrium intotal iron concentration prior to the start of the treatment. Themeasured total iron concentration is referred to as (Fe ppm UT) in theabove formula.

Acetoin, or 3-hydroxy-2-butanone was tested as a corrosion inhibitor inthe quick corrosion test apparatus with the procedure described above.Table I presents the percent corrosion inhibition obtained when acetoinwas fed to the iron coil in the test apparatus. The total ironconcentrations as measured by ICP were used to calculate the percentinhibition. The results demonstrated that the chemical was acting as ametal corrosion inhibitor. TABLE 1 % Corrosion Inhibition when Acetoinis fed % Corrosion Inhibition Run Acetoin (ppm) (ICP) 1 0 0.0 1 1 15.8 12 15.8 2 1 10.5 2 2 15.8 3 1 21.1

Riboflavin was also tested as a corrosion inhibitor in the corrosiontest apparatus following the same procedure as described above. Table IIpresents the results in percent corrosion inhibition of the metalobtained. The outcome demonstrated that riboflavin was also acting as ametal corrosion inhibitor. TABLE II % Corrosion Inhibition obtained withRiboflavin % Corrosion Inhibition Run Riboflavin (ppm) (ICP) 1 0 0.0 2 15.9 3 1 12.5 3 2 6.3 4 2 10.5 4 3 15.8

Diallyl disulfide was also tested as a corrosion inhibitor in thecorrosion test apparatus following the same procedure as describedabove. The chemical was emulsified with 20% polyoxyethylene sorbitanmonostearate. Table III presents the results in percent corrosioninhibition of the metal obtained. The outcome also demonstrated thatdiallyl disulfide was acting as a metal corrosion inhibitor. TABLE III %Corrosion Inhibition obtained with Diallyl Disulfide % Corrosion Diallyldisulfide Inhibition Run (ppm) (ICP) 1 0 0.0 1 1 78 1 2 65

Cysteine was also tested as a corrosion inhibitor in the corrosion testapparatus following the same procedure as described above; results arefound in Table IV, below. TABLE IV % Corrosion Inhibition obtained withCysteine % Corrosion Inhibition Run Cysteine (ppm) (ICP) 1 0 0.0 1 1 301 2 35 2 1 31.6 3 1 32.4 3 2 29.4

While this invention has been described with respect to particularembodiments thereof, it is apparent that numerous other forms andmodifications of this invention will be obvious to those skilled in theart. The appended claims in this invention generally should be construedto cover all such obvious forms and modifications which are within thetrue spirit and scope of the present invention.

1. A method of inhibiting corrosion on metallic surfaces in contact witha fluid contained in an industrial fluid system, which comprises addingto such fluid an effective corrosion controlling amount of a generallyrecognized as safe (GRAS) compound selected from the group consisting ofan acyloin, riboflavin, diallyl disulfide and cysteine.
 2. The method asrecited in claim 1 wherein said fluid system is selected from the groupconsisting of heat transfer systems, refinery systems, food and beveragesystems, and mechanical coolant systems.
 3. The method as recited inclaim 1 wherein said fluid system is a cooling water system.
 4. Themethod as recited in claim 1 wherein said fluid system is a steam headersystem.
 5. The method as recited in claim 1 wherein said compound isadded to the fluid system at active treatment levels ranging from about0.025 to about 50 parts per million.
 6. The method as recited in claim 6wherein said compound is added to the fluid system at active treatmentlevels ranging from about 0.05 to about 10 parts per million.
 7. Themethod as recited in claim 1 wherein said fluid system is a boiler watersystem.
 8. The method as recited in claim 1 wherein said fluid system isa gas scrubbing system.
 9. The method as recited in claim 1 wherein saidfluid system is a pulp and papermaking system.
 10. A method ofinhibiting corrosion on metallic surfaces in contact with a fluidcontained in an industrial fluid system, which comprises adding to suchfluid an effective corrosion controlling amount of a diallyl disulfidecompound.
 11. The method as recited in claim 10 wherein said fluidsystem is selected from the group consisting of heat transfer systems,refinery systems, food and beverage systems, and mechanical coolantsystems.
 12. The method as recited in claim 10 wherein said fluid systemis a cooling water system.
 13. The method as recited in claim 10 whereinsaid fluid system is a steam header system.
 14. The method as recited inclaim 10 wherein said compound is added to the fluid system at activetreatment levels ranging from about 0.025 to about 50 parts per million.15. The method as recited in claim 10 wherein said compound is added tothe fluid system at active treatment levels ranging from about 0.05 toabout 10 parts per million.
 16. The method as recited in claim 10wherein said fluid system is a boiler water system.
 17. The method asrecited in claim 10 wherein said fluid system is a gas scrubbing system.18. The method as recited in claim 10 wherein said fluid system is apulp and papermaking system.